1. Field of the Invention
The present invention is generally related to methods for leaching metal-bearing ores for the recovery of desirable metal values. More particularly, the present invention is directed to the leaching of such ores by improved heap leaching processes and improved heap construction.
2. The Prior Art
Desirable metals (such as gold, silver, copper, aluminum, uranium, and the like) are generally found as mineral constituents in naturally occurring ores. The most common method of separating the desirable metal values from the remaining undesirable constituents, often called the "gangue," is by chemical leaching of the ore, wherein ground or crushed ore is subjected to treatment with chemical solutions containing reagents capable of selectively solubilizing the desired metal constituents while leaving the gangue material intact. The leach solution is then treated in recovery and refining operations to obtain the metal values in a purified form.
The actual mechanism of leaching may involve simple dissolution made possible by administration of a suitable solvent, or, more commonly, involves dissolution made possible by a chemical reaction. The efficiency and rate of leaching depends upon many factors, including the rate at which the leach solution is administered, the amount of metal in the ore, and the conduciveness of the ore to leaching.
Some ores are quite permeable to leach solutions; hence, relatively large ore particles can be effectively leached. Many ores are, however, rather impermeable; as a result, the ore must be reduced to a small size before leaching in order to increase the surface area of the ore and to decrease the requirement for the leach solution to penetrate deeply into the ore particles.
Various methods of leaching metal ores have been developed, including the methods known as waste dump leaching, heap leaching, vat leaching, agitation leaching, and most recently, thin layer leaching.
Because of its gross inefficiency "waste dump" leaching has been used principally in connection with low-grade copper ores or pit wastes. The waste dump leaching method consists of stacking uncrushed ore into large, deep heaps (for example, 50 to 200 feet in depth) and percolating an acid and ferric sulfate leach liquor through the heaps so as to dissolve copper sulfide. The primary advantage of waste dump leaching is its low cost, which makes this method commercially feasible for use with low-grade ores despite its inefficiency in recovering the metal values from the ore. However, the inefficiency of the waste dump method makes it entirely unsuitable for use with higher-grade ores.
"Heap" leaching is a term used to describe a leaching process in which the ores are placed onto what is commonly known as a "pad." Generally, the pad consists of impermeable clay, and the crushed ore to be leached is stacked on the pad to a depth of between about 12 and about 30 feet. The ore is then leached by spraying a leach solution onto the top of the heap in order to create a downward percolation of the leach solution.
When leaching by percolation, the size of the ore particles is very important. If the particles are too large, the leach solution will not penetrate to the interior of the particles, and leaching is thus incomplete. Further, use of large particles typically results in a rapid percolation rate, thereby causing leach solution to pass through the heap too quickly. On the other hand, if the particles are too small, although the ore will be effectively penetrated by the leach solution, the percolation rate may become so slow as to be impractical.
The solution for dealing with particles that are too large for effective leaching is simply to reduce them in size. Conversely, undersize particles may be "agglomerated," such as by the addition of portland cement, in order to increase the percolation rate through the heap.
One serious problem that has plagued conventional heap leach processes is the difficulty in obtaining a uniform leach throughout the heap. Typically, the upper layer ore in such a heap is over-leached while the lower layer ore is inadequately leached.
Yet another problem when using a heap leach process is the difficulty in leaching the sides of the heap, especially when the heap consists of ores having low permeabilities or fine ores that are easily eroded. When leaching these types of ores, there is a tendency for the leach solution to run down the side of the heap rather than percolate through the heap.
In a heap leaching process, while the leach solution effluent is relatively rich in metal values initially, it often becomes quite weak as leaching continues over a period of weeks or months. This is particularly significant when it is realized that heaps of the type described above are typically leached for somewhere between a month and a year. The recovery facilities must be constructed so as to be capable of handling the relatively rich solutions obtained initially, even though this means that the recovery facilities are underutilized during the later period of time when the leach solutions become less concentrated with metal values.
After the ore has been leached to the maximum extent economically possible, there is the further problem of what to do with the tailings. One solution has been to leave the tailings in the heap and construct a new pad for additional recovery operations. However, in order to avoid building a multiplicity of pads, it is not uncommon to reuse a pad and to remove the tailings to a waste heap. Generally, when a pad is to be reused, relatively easily leached ore is heaped to a depth of only about 6 to 15 feet, and the heap is leached for a period of only about one to six weeks.
A different technique often used for leaching of ores, known as "vat leaching," typically involves the placing of crushed ore within a large concrete vat through which a leach solution is recirculated in an upward flow over a period of several days. Disadvantages of this method are that it is relatively expensive and also relatively inefficient. Because of these deficiencies, vat leach operations must be satisfied with recovering whatever metal values are leached in a few days, with the remaining metal values being discarded along with the tailings.
Agitation of the ore during leaching has been shown to significantly improve the recovery of metal values. Thus, another form of leaching, aptly termed "agitation leaching," is typically used for the leaching of high-grade ores wherein the substantially increased costs of this process are warranted. The agitation leaching technique typically involves the mechanical agitation of a slurry of very fine ore particles. This technique is extremely expensive in terms of capital and operating costs. First, the ore must be comminuted to a very small size before it will be efficiently leached in the short period of leaching utilized in this process. Additionally, the energy costs of agitating the slurry of ore particles are quite substantial. Nevertheless, because of its higher efficiency in terms of metal value recovery, this technique is often used on a commercial scale with high-grade ores.
Recently, another process for leaching ores, termed thin-layer leaching, has been developed. This process, which is described in U.S. Pat. No. 4,017,309, generally involves placing an acid-cured crushed copper or uranium ore onto a pad having a porous substrate. Unlike other heap leaching techniques, this technique involves placing the ore into a relatively thin layer (for example, less than two meters) and leaching the layer for a relatively short period of time (generally less than two weeks). Leach solutions are optimally routed to and from a number of pads so as to provide distinct stages of leaching and rinsing of the ores with leach solutions containing varying concentrations of reagent.
The thin-layer process has proven relatively effective in leaching many copper and uranium ores and permits a moderate reagent usage in comparison to the recovery of metal values. However, it has been found disadvantageous for use with slow-dissolving ores because of the small volume of ore processed in such a thin heap and the high costs of building pads for use in connection with this technique. This has substantially limited the use of the thin-layer process with ores containing gold and silver. It has also proven uneconomical for leaching ores containing primary uranium minerals or for leaching sulfide copper ores.
A disadvantage common to most of the foregoing leaching processes is the need for double-handling of the ore. Thus, the ore must be handled once when it is loaded onto a heap or into a vat, and it must be handled a second time when the ore is removed to a tailings waste pile.
Another disadvantage encountered when using one of the heap leaching processes is that leach solutions sometimes freeze during the winter months. As the leach solution is sprayed onto the top of the heap, the spray freezes, thereby shutting down the operation until the arrival of warmer weather. Even where freezing does not occur, low temperatures adversely affect the reaction rate of the leaching process. In many geographical areas, cold temperatures can result in a substantial nonproductive period. These problems of freezing and reduction of reaction rates have been alleviated somewhat by heating the leach solutions, but such an approach is expensive and has not proven entirely acceptable because the heat is dissipated rapidly as the solution is sprayed onto the heap.
From the foregoing, it will be appreciated that it would be a significant advancement in the art of leaching ores for the recovery of metal values to provide a process capable of use with a large variety of ores having differing characteristics of permeability, size, and leach characteristics. It would also be a significant advancement if a heap leaching process were to be provided that could obtain high concentrations of metal values in the leach solution, and maintain such concentrations relatively constantly, so as to allow optimum use of recovery facilities. It would be a further significant advancement in the art if heap leaching processes could be provided that was capable of low cost operation and would avoid the disadvantages presently encountered with side erosion of the heap, with winter-time operation, and with multiple handling of the ore.